Ask TennZen: High Acid vs. Low Acid Foods and Pressure Canning

Because of some of the questions I have received lately, I am under the impression that there are still a whole lot of people out there who are following unsafe advice when it comes to canning.

PLEASE - when it comes to canning foods, don't follow old recipes or word-of-mouth advice. Follow safe, lab-tested instructions from trusted sources like the National Center for Home Food Preservation.

Every canning technique and canning recipe that I post here has been tested and proven safe.

If you don't remember anything else about canning, please, PLEASE commit this rule to memory:

Low acid foods MUST be pressure canned. ALWAYS!

Class, let's repeat that:

Low acid foods MUST be pressure canned. ALWAYS!

Got it?

Good.

Now, how do you tell the difference between a low acid food and a high acid food?

Here's a good rule of thumb:

If it's a vegetable, then it's low acid. If it's a fruit or a tomato, then it's high acid.

VEGETABLE = LOW ACID
FRUIT / TOMATO = HIGH ACID

Are you still with me?

Let's address some frequently asked questions.

Q. Do I really have to use a pressure canner to can vegetables? I've heard of people canning beans and corn in a boiling water canner.

A. Yes, you need a pressure canner in order to safely can vegetables and meats. These are low-acid foods (pH higher than 4.6) and must be processed in a pressure canner.

Here's why: The main concern involves a bacterium called Clostridium botulinum. Though the bacterial cells are killed at boiling temperatures (212 degrees F / 100 degrees C), they can form spores that can withstand these temperatures. The spores grow well in low acid foods, in the absence of air, such as in canned low acidic foods like meats and vegetables. When the spores begin to grow, they produce the deadly botulinum toxins (poisons). These toxins cause botulism, which can be fatal.

The only guaranteed way to destroy the spores is to get the temperature up above 212 degrees F / 100 degrees C. It's impossible to do this with boiling water canning. The water will never get hot enough. We have to use a pressure canner, which raises the temperature up to 240 degrees F (about 116 degrees C). This is sufficient to kill the botulinum spores.

You may see some older recipes or hear that it's okay to process low acid foods in a boiling water bath, just as long as you process them for a very long time. Don't do it! The ONLY safe way to can low acid foods is with a pressure canner.

High acid foods - like fruits, jams, jellies, pickled foods and tomatoes, are too acidic for Clostridium botulinum to survive. It's okay to process these in a boiling water canner, as long as you follow the guidelines for correct processing time.

Q. I found your website too late. I already canned some vegetables in a boiling water canner. The jars sealed, though. Aren't they safe because of the airtight seal?

A. NO! Just because a jar has sealed doesn't mean that it's safe. It just means that there's no air in the jar and nothing from the outside can get in.

Botulism is a rare but serious paralytic illness caused by botulin toxin. Botulin toxin is one of the most powerful known toxins: about one microgram is lethal to humans. The toxin is produced by the bacteria Clostridium botulinum. C. botulinum is anaerobic, meaning that it lives in an environment that is absent of oxygen. In other words, it thrives in a vacuum, just like a sealed canning jar.

It's not an airtight seal that kills C. botulinum and protects your food, it's processing at high temperature - higher than that of boiling water alone. Only a pressure canner can raise the temperature up to 240 degrees F (about 116 degrees C), which is hot enough to kill the botulinum spores.

Q. So, what do I do with all these jars of vegetables that I canned in a boiling water bath?

A. It depends on how long ago you canned them.

If you canned them less than 24 hours ago, you can store the food in your refrigerator and eat it within the next few days. No more than a week.

Or, you can still salvage the food by freezing. Empty the jars of food into a large pot. Heat the food for at least 10 minutes. Then pack the food into freezer bags or boxes and freeze.

If you have a pressure canner available, you can try to salvage the food by pressure canning. The food quality may be diminshed, though. Beans may be mushy. Empty the jars of food into a large pot and heat to boiling. Discard jar lids. Wash, rinse and reheat jars and rings. Wash, rinse, and heat new lids. Prepare pressure canner for processing. Pack food into clean jars, leaving 1 inch of headspace. Place on new lids and rings and place jars in canner. Process in pressure canner at correct time and pressure according to the type of food being canned. Refer to the USDA Complete Guide to Home Canning for correct processing times and pressures.

If it's been more than 24 hours since you canned the food in a boiling water canner and the jars have NOT been stored in a refrigerator, the food is potentially unsafe and must be discarded.

Q. I have a pressure cooker - it doesn't say that it's a pressure canner. Is it okay to can in my pressure cooker?

A. NO. A pressure cooker is different from a pressure canner. One easy way to tell the difference is that a pressure cooker usually has a rounded bottom. A pressure canner has a flat base. Another way to tell is by height and volume. A pressure canner is tall enough to accommodate a quart jar and have at least 1 inch of space above it.

Pressure cookers heat up and cool down more quickly than pressure canners. Because of this, pressure cookers cool down too fast, which would cause any foods canned inside them to be underprocessed. The heat up/cool down time is factored into safe processing times for pressure canner recipes.

You can only process foods safely in a pressure canner - specifically designed for canning.

If you are ever in doubt about something, look up the answer from a trusted source before you start canning.

And when it comes to eating canned foods... when in doubt, throw it out. Don't take chances.